A vacuum pump having a turbo-molecular pump part as described in Japanese Patent Laid-Open No. 2003-269364 (“JP '364”)(See scope of claims for patent, paragraph numbers 0021 and 0034, FIG. 2, FIG. 3, FIG. 4), for example, is known. JP '364 discloses a vacuum pump where spacers (50) are positioned between the stator blade wheels (11, 11) of upper and lower stages to separate (or create a gap between) the stator blade wheels (11, 11) at a prescribed distance. The spacers (50) have an outer circumferential part (50a) in contact with an inner circumferential part (1a) of a pump case (1). Similarly, each stator blade wheel (11) has an outer circumferential part (11a) that is also in contact with the inner circumferential part (1a) of the pump case (1). Alternatively, the outer circumferential part (11 a) of the stator blade wheel (11) may also be in contact with part of the spacers (50). JP '364 overcomes the problems in the prior art by using spacers (50) having a relatively simple geometry which results in cost reductions.
The stator blade wheel 11 of JP '364 includes a plurality of blades that are radially arranged and integrally connected via two inner and outer flanged parts (11-1, 11-2) having the shape of a semicircular arc. There is also a stator blade wheel of another construction without the outer flanged part (11-1). Though not described in the Detailed Description, the drawings show that the stator blade wheel (11) has a simple circular disc shape as viewed from the side, and the thickness of the two inner and outer flanged parts (11-1, 11-2) is the same as the thickness of a part where a blade is provided, and the whole provides flat upper and lower surfaces.
JP '364 does not describe the setting of the gap between the stator blade wheels (11, 11) in detail. However, the gap between the stator blade wheels (11, 11) is typically set to ensure that an appropriate gap between a rotor blade wheel (10), which is positioned between the stator blade wheels (11, 11), and the stator blade wheel (11) is sufficient. During manufacturing and assembly, small variations in the thickness of the individual spacers and stator blade wheels can accumulate. For this reason, it is important to appropriately control the spacing of all gaps between the rotor blade wheel. (10) and the stator blade wheel (11). Thus, the apparatus of JP '364 requires precise machining of the thickness of the spacers and stator blade wheels and elaborate assembly.
As described above, the stator blade wheel (11) has a plurality of blades that are radially arranged and integrally connected via the inner flanged part (11-2) having the shape of a semicircular arc or connected additionally by the outer flanged part (11-1). Each radial blade is relatively thin-walled and is susceptible to deformation by an external force. Therefore, without the outer flanged part (11-1), deformation is likely to occur from compression between the spacers during pump assembly and it is difficult to ensure that the gap between the stator blade wheels is set at a prescribed value.
While attaching the outer flanged part (11-1) to the stator blade wheel (11) eliminates the problem of deformation, another problem arises with the accuracy of the spacer-abutment face of the outer flanged part. In general, because of the complex shape of the radial blade part, the stator blade wheel is formed from an aluminum alloy using precision casting and the like. Precision casting and other forming methods cause the upper and lower spacer-abutment faces of the outer flanged part to be rough, and the thickness accuracy is insufficient. Therefore, the spacer abutment face is unable to perform its function. For this reason, finish machining by cutting, grinding and the like must be further performed to obtain a prescribed thickness and even contact between the upper and lower spacer-abutment faces.
Because JP '364 is not directed to setting the gap between the stator blade wheels, there is no explanation in the detailed description about setting the gap between the stator blade wheels and the section of the stator blade wheel in the drawings is drawn in a simple plate-like shape. That is, in JP '364 there is no description of a stator blade wheel in which the spacer-abutment face is easily finish machined.
FIGS. 8(a), 8(b) and 8(c) show conventional vacuum pumps having stator blade wheels suitable for finish machining the upper and lower spacer-abutment faces of the outer flanged part.
In FIG. 8(a), a plurality of rotor blade wheels 4a integrally formed on a rotor (not shown) and a plurality of stator blade wheels 12 provided within a cylinder of a pump case 11 are alternately arranged with a prescribed gap g1. The stator blade wheels 12 are held in a multi-stage manner by ring spacers 13 with a prescribed gap. An outer ring part 12a is present on the outer circumference of the stator blade wheel 12 and upper and lower end faces thereof provide base end faces 12tb, 12ta. The base end faces 12tb, 12ta abut against a lower surface 13f and an upper surface 13e of the ring spacer 13.
The outer ring part 12a is thicker than the blade part of the stator blade wheel 12 by an amount corresponding to a level difference s, which is sufficient for performing finish machining. Because of this level difference s, the finish machining of the base end faces 12tb, 12ta can be safely performed without contacting the finish machining tool with the blade part of the stator blade wheel. To further ensure that the finish machining tool does not contact the stator blade part, the small gap g1 between the rotor blade wheel 4a and the stator blade wheel 12 has been increased further by f, thus increasing the level difference s.
With this structure, the base end faces 12tb, 12ta are thus able to be finished with the same level of machining accuracy and finishing as the ring spacer 13 and, in addition, the thickness of the outer ring part 12a can also be machined with good accuracy. As a result of the spacer 13 and stator blade wheel 12 having precisely machined base end faces, the axis line gap g1 between the rotor blade wheel 4a and the stator blade wheel 12 can be set at an appropriate value.
An outer circumferential face 12ac of the outer ring part of the stator blade wheel 12 abuts against an inner cylinder face 11b of the pump case 11 and fixes the radial position of the stator blade wheel 12, as a result, the radial gap g2 between the rotor blade wheel 4a and the stator blade wheel 12 is set at an appropriate value.
The smaller the axis line direction gap g1 and radial gap g2 between the rotor blade wheel 4a and the stator blade wheel 12, the better the pump performance. However, an appropriate axis line direction gap g1 and an appropriate radial gap g2 are necessary in order to prevent: 1) the rotor blade wheel 4a from being instantaneously deformed during the rotation of the rotor blade wheel 4a due to gas and the like entering the pump and the rotor blade wheel 4a; and 2) the stator blade wheel 12 from contacting the rotor blade wheel 4a due to machining errors of the rotor blade wheel 4a, stator blade wheel 12 and spacer 13, pump assembly errors and the like.
In a vacuum pump of this kind, as described also in JP '364, the stator blade wheel 12 is divided into two semiannular parts in order to permit pump assembly. The annular stator blade wheel is cut into halves using a tool. As a result of this cutting, the stator blade wheel is ground by a cutting width w that substantially corresponds to the width of the tool and only a linear portion having the cut width w is cut from the circular middle part (see FIG. 3).
On the other hand, in a standard form, each stator blade wheel 12 obtained by combining two semiannular parts abuts snugly against the inner cylinder face 11b of the pump case as shown in FIG. 8(a). However, because a gap corresponding to the cutting width w is present between the two semiannular parts, in the pump assembling process, there is a possibility that due to a radial shift the stator blade wheel 12 departs a little from the inner cylinder face 11b of the pump case, thereby causing problems illustrated in FIGS. 8(b) and 8(c).
FIG. 8(b) shows that the stator blade wheel 12 shifts by an amount w′ smaller than the cutting width w, with the result that the rotor blade wheel 4a and the stator-blade-wheel outer ring part 12a approach k′. In this condition, there is a possibility that the outer circumference of the rotor blade wheel 4a and the inner circumference of the stator-blade-wheel outer ring part 12a will come into radial contact with each other.
FIG. 8(c) shows that the stator blade wheel 12 shifts by the cutting width w and that the rotor blade wheel 4a and the stator-blade-wheel outer ring part 12a approach each other, with the result that a corner of the rotor blade wheel 4a interferes with a corner of the stator-blade-wheel outer ring part 12a by f in the axis line direction and by k in the radial direction, making assembly impossible.
To prevent the problems illustrated in FIGS. 8(b) and 8(c), it is required that the cutting width w of the stator blade wheel 12 be as small as possible. Such cutting has conventionally been carried out by wire electric discharge machining and the like using a wire that provides a fine cutting width. However, this wire electric discharge machining requires a long machining time and is costly. A practical cutting method that provides a small cutting width w and that is efficient is needed.